Method of preparing lithographic printing plates



3,133,498 METHOD OF PREPARING LITHOGRAPHIC PRINTING PLATES Michael Michalchik, Levittown, N..I., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Filed Mar. 14, 1962, Ser. No. 179,776

- 3 Claims. (Cl. 101--l49.2)

This invention relates to-improved lithographic printing plates and, more particularly, to the use of improved conversion compositions -for preparing lithographic printing plates from prints made by an electrostatic duplicating process.

Lithographic printing is a well known form of planographic printing. In general, the process includes printing from a flat .(or planograp-hic) plate and it depends upon different properties of the image and non-image areas for printability. In the ordinary lithographic printing process, a printing plate is prepared by affixing to a water-attractive, hydrophili'c surfacea Water-repellent hydrophobic image, usually greasy,fresinous or waxy in nature. An aqueous wet-out liquid .is then applied'to the surface so imaged. This liquid wets all portions of the surface not occupied by the hydrophobic image material but does not wet the hydrophobic image.

An inking roll coated with a grease-base lithographic printing ink is passed acrossthe surface of the printing plate, leaving a 'filrn' of ink upon the ink-receptive hydrophobic image areas. No ink is left on the non-image areas which are covered by a film of ink-repelling wetout" liquid.

When the inked'plate is'brought into contact with another ink-receptive surface. some of the ink from the image areas transfers thereto. The ink image may thus be transferred directly to a paper sheet but generally is transferred, first, to a rubber offset blanket which, in turn. is used to transfer the print to a final paper sheet. For each print made during a printing run, the printing plate is dampened with the wet-out or, as it is usually called, the fountain solution, for the purpose of keeping the non-image areas wet.

In the past, planographic printing plates have been made of zinc or aluminum sheets having surfaces especially treated to render them hydrophilic. For shorter printing runs, paper base plates coated with a pigment dispersed in a hydrophilic adhesive, have been used.

Images have generally been placed on the metal plates photographically withthe aid of a special photographical- 1y sensitized master, or, non-photographically, as with a grease pencil. Images have generally been placed on the paper base plates by typing using a special ribbon.

More recently, electrostatic methods have been used for applying images to lithographic printing plates. This type of process, in general, comprises placing an over-all, uniform electrostatic charge on a plate having a surface made of a photoconductive material, such as selenium, exposing the plate to an image of light and shadow to be reproduced, thereby removing the electrostatic charge in those areas'of the plate receiving light, and thus producing an electrostatic charge pattern on the plate surface exactly corresponding to the dark areasof the original to be reproduced. The charge pattern is made visible by contacting the surface bearing the electrostatic charge pattern with a finely divided electroscopic developer material (also referred to herein as-a toner") bearing a charge opposite to that of the electrostatic image, whereby the developer material deposits on the plate in image configuration.

The powder image is then transferred to the printing plate surface and stabilized as by heating to a temperature sufficiently high to, fuse thedeveloper material and cause it to adhere to the plate surface.

3,133,43 Patented May 19, 1964 Still more recently, the above-described electrostatic method has been improved by eliminating the transfer step necessitated by using a selenium plate to first record the light image. In the improved method the image is recorded and fixed directly upon an improved type of printing plate. The improved'type of printing plate has-a printing surface composed of'a powdered photoconductive material, preferably a particular grade of photoconducting zinc oxide, and a resin binder. The resin binder must have good electrical insulating properties since the plate must be able to hold an electrostatic charge for an appreciable length of timein darkness. It has further been found that resins which are hydrophilic are unsuitable since they apparently attract sufficient water to lower the 1 surface resistance to. an unsuitable level. This permits too much lateralleakage of the electrostatic charge and prevents storage of an electrostatic charge'pattern long enough to permit satisfactory developing; Therefore heretofore the resin binders used in the above-described recording composition have been limited to those which are hydrophobic.

As previously pointed out, a lithographic printing plate surface must have hydrophilic properties in its non-printing areas. Thus, in order to use a conventional zinc oxide-resin coated electrostatic recording surface as a lithographic printing plate, the hydrophobic surface must first be converted to hydrophilic. Solutions for accomplishing this conversion have been devised and have been used with more or less success. Some of these, however, require an unduly long treating time and do not work satisfactorily unless the plate is completely immersed in the solution for an appreciable period of time. It is desirable to be able to accomplish the conversion step in a highly effective manner but in as short a time as possible and with as simple a treating technique as possible. It is also desirable to use a conversion solution which has no decomposable organic substances and yet is simple and economical to formulate.

In the case of zinc oxide-resin coated plates, the conversion composition produces an insoluble substance on the surface of the zinc oxide particles that attracts water to its surface. It is desirable for a satisfactorily acting conversion composition that it shall not undercut the fused toner deposit on the printing surface or weaken the deposit in any other manner.

One object of the present invention is to provide im-' proved conversion compositions for converting a zinc oxide-hydrophobic resin surface on a planographic print- '1 ing plate from hydrophobic to hydrophilic properties.

Another object of the invention is to provide improved conversion compositions, for use on planographic printing plates having zinc oxide-hydrophobic resin non-printing areas, which are simple and economical to formulate and which are highly stable over extended periods of time.

Still another object is to provide planographic plates for lithographic printing having improved printing images,

wherein the non-image areas of the plates are composed of zinc oxide and a hydrophobic resin.

In the present invention, the above and other objects are attained using a conversion composition consisting essentially of a soluble tin compound, an alkali thiocyanate, a strong mineral acid and water. Alternatively, the improved composition may comprise a paste of stannous hydroxide. In either case water-insoluble substances are deposited on the treated surface of the plate.

Following are examples of utilizing the improved conversion compositions of the present invention.

Example I A planographic plate is prepared by spray-coating an aluminum sheet to n thicl tncss of about 1.0 mil (after drying) with the following composition:

Pliolite S-7 (butadiene-styrene copolymer of the Goodyear Tire and Rubber Company) gm 270 SR-82 (silicone resin of the General Electric Company gm 135 Zinc oxide (Green Seal #8, New Jersey Zinc compa y). g n-- 800 Xylene rn1 2,000 Dye (2% solution of sodium fluorescein in methanol) ml 11 face is exposed to a positive image pattern of the subject matter which is to be printed so that a charge image of the subject matter remains on the surface of the printing plate.

Then the charge image (again in the dark) is developed by sweeping across the surface a mixture of iron particles free from grease and other contaminants and having a greatest dimension between about 0.002 and 0.008", and an easily fusible powder or toner above iron in the triboelectric series. The iron serves as a carrier for the toner particles. Particles of toner are attracted to the electrostatic charge image and deposit in the charged areas.

A suitable toner can be made by, first, melting then cooling and ball milling together 200 g. of Piccolastic resin 4358A (Pennsylvania Industrial Chemical Co.), 12 g. carbon black G, 12 g. Nigrosine SSB dye and 8 g. Isol Black dye. The ground powder is finally screened through a 200 mesh screen.

The toner particles are fused to the zinc oxide-resin coated surface by heating with an infra-red lamp for about 20 seconds at 190 C. This completes the formation of the printing image.

Using the printing plate as described above, a planographic printing run is carried out as follows.

A conversion solution in accordance with the present invention is made up with the composition:

Stannous chloride (SnCl .2H O) g 2 Potassium thiocyanate (KCNS) g 3 Hydrochloric acid (Conc.) ml 1 Water ml 6,000

Any precipitate which forms is filtered off and discarded.

In this example, any soluble tin salt may be used, either stannous or stannic. Any alkali thiocyanate such as sodium or'lithium may also be used. The thiocyanate appears to aid the penetration of the tin salt into the surface of the printing plate. The amount may be varied from about 1-3 g. per 6 liters of water for most satisfactory results. The hydrochloric acid provides a stable solution and may be varied in amount somewhat, but if too little is used, the solution becomes unstable prior to use. Other strong mineral acids may also be used.

The clear conversion solution is swabbed on the imagecontaining surface of the plate so that the entire surface is thoroughly and uniformly wetted. The solution is allowed to react upon the zinc oxide-resin surface for 15- 20 seconds. The stannous salt forms water-insoluble substances on the zinc oxide particles providing them with a surface having the desired hydrophilic properties.

In order to increase contrast in the printed product, the plate is next treated with a solution of Platex (Addressograph-Multigraph Corp., Cleveland, Ohio). Platex is believed to comprise a nickel salt, ammonium acid phosphate, butanal, cellulose gum, diethylene glycol, formaldehyde, and water. The solution is applied in the form of one volume Platex to 3 volumes water. This solution 4 reduces any tendency of the conversion solution to wet the black areas of the printing surface.

The next step in the process is to swab the printing surface of the plate with the fountain solution to be used on the press during the printing run. Sutficient fountain solution is applied in this manner to wash off the Platex.

The plate is mounted on a lithographic press and wet with a fountain solution each time a print is made. A suitable fountain solution comprises:

Sodium ferrocyanide 0.1% by weight.

Glacial acetic acid 0.5% by volume.

Glycerine 1.0% by weight.

Water balance.

Example I! As in Example I, a printing plate was made up by coating an aluminum sheet with the following composition:

Pliolite S-5 g 77 Pliolite S-7 g 270 SR-82 g 5.0

Zinc oxide (Green Seal #8) g 800 Xylene ml 2,000 Dye (2% solution of sodium fluorescein in methanol) ml 11 Parts by weight Piccolastic Resin D- 60 Piccolastic Resin C- 40 Carbon black 7 Calco Black Dye (F-4160B, Allied Chemical and Dye Corp.) 2

The resin is melted'and the carbon black added to the melt, followed by the dye. The ingredients are thoroughly mixed at a temperature of about 175 200 C. The mixture is then cooled and the hard cake is crushed and screened to -20 mesh. This powder is micronized to an average particle size of 5-10 microns.

After this toner is applied to the charge image, it can be fused to the zinc oxide-resin surface by heating to the relatively low temperature of C. for 30 seconds.

A conversion composition for this printing surface is made up as follows: Five grams of SnCl .2H O is dissolved in 90 ml. of water. Five ml. of concentrated ammonium hydroxide is added to this solution. The addition of the hydroxide causes a precipitate of stannous hydroxide to form. After allowing the precipitate to settle, most of the liquid is decanted leaving a paste. The amount of ammonium hydroxide should be enough to provide excess ammonia and render the paste alkaline.

The paste of stannous hydroxide is applied evenly over the surface of the printing plate by swabbing using a cotton swab. After permitting the conversion composition to react with'the non-image areas of the printing surface for about 10 seconds, the remainder of the composition is thoroughly rinsed away with water. As in the previous example, the stannous compound forms an insoluble but hydrophilie substance on the surface of the plate.

The thus converted printing surface may be used to make a printing run on a lithographic press using only distilled water as a fountain solution and without any A intermediate treatment as in the previous example.

The improved conversion compositions of the present invention may be varied somewhat within the scope of the invention.

The hydrophobic resin used as the binder with the zinc oxide of the printing plate is not critical. However, the

resin binder must not be hardened to the extent that the conversion solution cannot penetrate to act on the zinc oxide.

What is claimed is:

1. A- planographic printing plate having non-image areas comprising zinc oxide and a hydrophobic resin binder, said areas having been rendered hydrophobic by treating with an aqueous solution comprising about 2 grams of a solubletin salt and about 13 grams of an alkali thiocyanate per 6000 ml. of water, said solution non'image areas hydrophilie which comprises treating said areas with an aqueous solution comprising about 2 grams stannous chloride and about 1-3 grams of potassiu'm thiocyanate per 6000 ml. of water, said solution containing just sufficient hydrochloric acid to render it stable.

3. In the method of planographic printing wherein the printing plate has non-image areas comprising zinc oxide and a hydrophobic resin hinder, the step of rendering said non-image areas hydrophilic which comprises treating said areas with a paste of stannous hydroxide.

References Cited in the file of this patent Australia Sept. 12, 1957 

1. A PLANOGRAPHIC PRINTING PLATE HAVING NON-IMAGE AREAS COMPRISING ZINC OXIDE AND A HYDROPHOBIC RESIN BINDER, SAID AREAS HAVING BEEN RENDERED HYDROPHOBIC BY TREATING WITH AN AQUEOUS SOLUTION COMPRISING ABOUT 2 GRAMS OF A SOLUBLE TIN SALT AND ABOUT 1-3 GRAMS OF AN ALKALI THIOCYCNATE PER 6000 ML. OF WATER, SAID SOLUTION CONTAINING JUST SUFFICIENT STRONG MINERAL ACID TO RENDER IT STABLE. 